1. Field of the Invention
The present invention relates to a technology for inspecting presence or absence of a foreign matter or a defect on a sample such as a wafer or a disk medium.
2. Description of the Related Art
Along with a tendency toward high-density patterning of electronic devices, the minimum line width of a patterned shape included in a large-scale integration or the size of a recording bit on a magnetic disk has been reduced down to an ultramicroscopic size of 100 nm or less. The tendencies toward the ultramicroscopic designing and high-density patterning would further continue. In the process of such high-density patterning, the presence of a small foreign matter (particles or scratches) of about 10 nm in size, which has not posed a problem previously, and a defect such as imperfect patterning causes a large defective. For example, when a pattern requested to exhibit a precision of 20 nm is formed, if a foreign matter of about 10 nm in size exists from the beginning, imperfect patterning caused by the foreign matter is of a level adversely affecting product quality. Consequently, in order to improve and sustain a product yield, the presence of such ultramicroscopic foreign matters and defects should be inspected prior to patterning, and be discriminated prior to a patterning process.
As a method of detecting such an ultramicroscopic foreign matter or defect, there is a method employing a scanning electron microscope (SEM) that sweeps a focused electron beam. For example, an SEM appearance inspection apparatus including an image processing unit that compares an acquired SEM image is employed (refer to, for example, Japanese Patent Application Laid-Open No. 05-258703). However, since the SEM appearance inspection apparatus two-dimensionally sweeps an electron beam over the surface of a sample so as to form an image, the SEM appearance inspection apparatus has a drawback that the inspection time is long. The long inspection time hinders introduction of the SEM appearance inspection apparatus into a production line in a factory for mass production of semiconductor chips or magnetic disks.
Aside from the issue of the inspection time, what impedes prevalence of an inspection apparatus employing an electron beam is an issue of charging caused by use of charged particles, that is, an electron beam. In the SEM, secondary electrons generated by an electron beam (a primary electron beam) are used to form an image. The secondary-electron generation efficiency varies depending on the energy of primary electrons or a material that emits the primary electrons. Moreover, the efficiency may exceed 1 or fall below 1. Consequently, charge applied to a specimen by primary electrons or electrons carried away by secondary electrons may be too excessive or too short. If the specimen is conducting, a significant problem will not occur. However, if the specimen is an insulator, whether charge to the specimen is too excessive or too short brings about positive or negative charging of the surface of the specimen. Charging on the surface of the specimen causes a variation in brightness of a secondary-electron image or a distortion thereof.
As mentioned above, for adaptation of an inspection apparatus that utilizes an electron beam in order to cope with the tendency toward the microscopic designing of electron devices, improvement in an inspection speed and control of charging lie as significant technological issues. In recent years, novel electron-beam applied inspection methods characteristic of a high inspection speed have been applied (refer to, for example, Japanese Patent Application Laid-Open Nos. 11-108864 and 2005-228743). The inspection technologies are such that after a negative potential close to an acceleration voltage for an electron beam to be irradiated to a sample is applied, the electron beam is irradiated to an entire inspectional field of view on the sample, and electrons reflected from the sample are focused in order to form an electron image for inspection. However, although reflected electrons are said to be focused, the technology disclosed in Japanese Patent Application Laid-Open No. 11-108864 is an inspection technology to which a mirror electron microscope (MEM) is applied so that a slightly more negative potential than an acceleration voltage for an irradiated electron beam will be applied on a sample, and electrons reflected due to the negative voltage will be imaged without colliding against the sample surface. The technology disclosed in Japanese Patent Application Laid-Open No. 2005-228743 Patent Reference 3 is an inspection technology to which a low-energy emission electron microscope (LEEM) is applied so that a more positive potential by, for example, 20 V or less than an acceleration voltage for an irradiated electron beam will be applied on a sample, the irradiated electron beam will collide against the surface of the sample with low energy, and electrons reflected from the sample surface will be imaged.
Incidentally, reflected electrons mentioned in this specification shall include both mirror electrons and low-energy emitted electrons.
On the other hand, as a control technology for charging caused by an electron beam, technologies for canceling charging that occurs during observation of an insulator sample using an SEM have been proposed (refer to, for example, Japanese Patent Application Laid-Open Nos. 10-312765 and 2002-231172). In the conventional technologies, ultraviolet rays are irradiated in order to make the insulator surface conductive so as to cause charge to flee. Otherwise, ultraviolet rays are used to generate photoelectrons so as to change the surface in a direction of positive charging, and a negatively charged insulating film is thus neutralized in order to cancel charging on the insulator surface.